Sound masking can diminish the performance impairment due to background speech in open-plan offices. This paper compares a steady-state masking sound with the spectrum of the disturbing speech signal to a time-reversed speech masker. As part of a laboratory experiment subjects have to complete a digit span task and a questionnaire. Both masking sounds improve the number recall performance as compared to unmasked speech. When the speech-to-noise ratio is reduced, the error rates decrease only during stationary sound masking. Sound masking with time-reversed speech increases the speech privacy at higher speech-to-noise ratios but it is perceived as more annoying.